Junior hospital doctors make mistakes in about 8% of the prescriptions they write for hospital patients according to a study commissioned by the General Medical Council (GMC). In the 19 hospitals surveyed, there were more than 11,000 mistakes, of which about 220 were potentially lethal. People were prescribed drugs they were allergic to, dangerous combinations of drugs and wrong doses. On the plus side, they got it right 92% of the time and most errors don't cause serious harm.

If you are admitted to hospital, the greatest danger you face is falling out of bed or on a slippery floor. The next most common cause of harm in the NHS comes from errors in prescribing. Only the intervention of vigilant pharmacists stops the figure being even higher.

I'm amazed that the study for the GMC found only 8% of errors. Hospital drug charts are complex, handwritten affairs that vary from hospital to hospital. There are different sections for once only, regular, intravenous and drugs to take home on discharge. Handwriting ranges from the very clear to the totally illegible. By the end of hospital stay, the drug chart can be a mass of crossings out, changes and corrections. Any doctor prescribing a drug needs to take into account allergies, interactions with other drugs, concurrent diseases (such as impaired kidney or liver function) and sometimes the age and weight of the person.

Prescribing is a minefield. For a start, all drugs have at least two names: the generic (chemical) name and different proprietary names given by the drug company. Hospital pharmacies have their own formularies and may not stock the same drug that a patient comes into hospital on, so doctors need to know about which drugs are equivalent. Converting people from one drug to another, such as the liquid morphine preparation Oramorph to long-acting morphine sulphate, requires a recalculation of doses. The maths is straightforward and medics are supposed to be numerate but in the middle of the night or on a busy ward round, it's easy to get it wrong.

Junior doctors don't operate in a vacuum; more senior doctors, nurses and pharmacists should provide a checking system. And the most important resource of all in avoiding errors is, of course, the patient who is best placed to remind doctors of their allergies and the fact that last time they had that drug they nearly died. But it's easy to see how errors happen when patients may not be at their most alert (or even conscious), nurses are busy and pharmacists not on hand to watch the administration of every drug.

Interestingly, in this study, first-year junior doctors made fewer errors than their second-year counterparts, presumably because the fledglings are terrified of getting it wrong whereas after a year in the job, they get a bit cocky.

General practice tends to be safer, though we GPs still make plenty of mistakes. We are less likely to initiate treatment with potentially lethal drugs (although even the humblest of drugs, such as paracetamol, can be lethal). More importantly, the vast majority of our prescriptions are now computerised. The computer software that GPs use flashes up warning messages about interactions. It asks probing questions ("Are you sure you want to issue this?"). It even gives you a hard time if you try to prescribe quantities of drugs that don't conform to the standard pack size. The patients' allergies and concurrent conditions are easy to view. And the prescribing bible, the British National Formulary (BNF) is online and just a click away to check doses, side effects and contra-indications.

People who are seeing a doctor or being admitted to hospital are well advised to write down any allergies and past bad experiences with specific drugs. A list of current medication and conditions can't hurt either. It doesn't pay to assume the doctor will know, remember or appreciate the significance of medicines you are prescribed.

If in doubt, question the doctor's prescribing. Don't worry about the doctor who looks things up in the BNF. Worry about those that don't.